The present invention relates to the construction of pressurized tanks of considerable size, such as the protective envelopes of nuclear reactors for nuclear power plants, gas holders and other containment structures having to withstand pressure of the order of 5 atm, and, more particularly to a wall of a pressurized reinforced concrete tank.
One of the main requirements of nuclear power plants is the guarantee of their safety during operation. The most widely used and reliable method of ensuring this requirement is the installation of a nuclear reactor and systems of the first (radioactive) circuit in a sealed protective envelope. In case of rupture of a pipeline or other failure in the first circuit, accompanied by leakage of heat-transfer medium, a gauge pressure is built-up within the protective envelope, and the envelope thus becomes a pressurized tank. It should be noted that not only must the gauge pressure be absorbed by the envelope walls, but sealing of the walls must be ensured.
With the growth of power output of nuclear reactors, the quantity and parameters of heat-transfer media increase, and the pressure developed within the protective envelope in emergency situations also increases.
At present, the main constructional material for protective envelopes of nuclear reactors having a power output of 500 MW and more is prestressed concrete. Envelopes for nuclear reactors which are made of prestressed concrete and which have cylindrical walls, a flat bottom wall and a flat or arcuate covering have found wide application.
The wall concrete is generally prestressed by the use of annular (transverse) and longitudinal reinforcements placed in ducts formed in the wall. The protective envelopes in reactors of nuclear power plants in the USA and Canada have such a construction.
In the known protective envelopes, tensile stresses are developed over the entire cross-section of the wall. These stresses may cause the formation and opening of cracks upon an increase in pressure, and result in an unsealing of the protective envelope. Unsealing of the protective envelope of a nuclear reactor during emergency conditions is extremely dangerous since radioactive pollution of the environment may occur.
Prestressing of the envelope wall improves the cracking resistance of the structure. However, due to errors in the prediction of maximum pressure value in the tank under emergency conditions, as well as due to other factors, the prestressing cannot completely prevent the cracks from opening.
Known protective envelopes are cast in place. The introduction of industrial methods to the construction of pressurized tanks by using prefabricated reinforced concrete members requires the prestressing of joints between the prefabricated members forming the walls thereof. Prestressed joints tensioned by means of bolt fasteners appear to be most promising since they may be assembled using portable power tools. Known flange bolt joints for joining prefabricated members including flanges and tensioning bolts are rather unsuitable because, in the course of operation, access to the bolts mounted at the inside part of the wall for inspection and tightening is difficult or even absolutely impossible.